The necessity for obtaining smooth and planar surfaces for the various layers in the production of multi-layer semiconductor wafers is well documented. Such layers are polished to the desired surface characteristics by a variety of different techniques. Chemical-mechanical polishing (CMP) is presently the technique of choice.
CMP employs a disk-shaped rotating platen, with a polishing pad, which rotates about an axis. In copending application serial numbers YUEH-1 filed Jan. 29, 1997 for the inventor of the present application, an annular-shaped platen with a non-rotating center core is disclosed. In either case, a wafer is secured to a sensor motor which rotates the wafer about its own axes and advances the wafer into contact with the pad. Thus, both the wafer and the platen are rotating and are in contact at a pressure determined by the actuator. The frequency at which the wafer rotates and the pressure on the wafer will be seen to be important parameters herein. A slurry is introduced between the wafer and the pad to aid in the polishing operation.
It is important to remove a sufficient amount of material to provide a smooth surface without removing an excessive amount of underlying materials. Consequently, it is important to monitor the removal rate or the wafer thickness variations and to ascertain when the end point of the polishing,, process has occurred.
There are many patents directed at techniques for determining that end point. U.S. Pat. No. 5,240,552, issued Aug. 31, 1993 describes an acoustic technique for determining such an end point. U.S. Pat. No. 5,413,941, issued May 9, 1995 describes a laser technique for determining the end point. U.S. Pat. No. 5,433,651 describes an optical technique using reflectance of light directed at the wafer through an aperture in the platen. All such techniques require sophisticated monitoring and control apparatus and achieve, at best, end point termination accuracy of about five hundred angstroms.